A lithium-ion power supply unit that employs lithium-ion cells as a power source (such a power supply unit is also called a "lithium-ion battery pack") usually incorporates a lithium-ion cell protection circuit for protecting the lithium-ion cells from overcurrent, overdischarge, and overcharge. A conventional lithium-ion power supply unit incorporating, for example, two serially connected lithium-ion cells and a lithium-ion cell protection circuit is constituted and operates as follows.
FIG. 4 is a block diagram of a conventional lithium-ion power supply unit having two lithium-ion cells connected in series. Numerals 101 and 102 represent lithium-ion cells. Numeral 103 represents a charge control FET (an n-channel field-effect transistor) for switching the state of the power supply unit between a state in which the lithium-ion cells 101 and 102 are charged and a state in which they are not charged. Specifially, when the FET 103 is on, the lithium-ion cells 101 and 102 are charged, and, when the FET 103 is off, they are not charged. Numeral 104 represents a charge control FET control circuit for turning on and off the charge control FET 103. Numerals 105 and 106 respectively represent a positive and a negative output terminal of the power supply unit. Numerals 107 and 108 represent resistors. Numeral 109 represents a chip capacitor provided as a capacitance element. Numeral 200 represents a lithium-ion cell protection circuit for protecting each of the lithium-ion cells 101 and 102 from overcharge. T1, T2, T3, T4, and T5 represent terminals of the lithium-ion cell protection circuit 200.
The higher-potential end of the series of the two lithium-ion cells 101 and 102 is connected to the positive terminal 105, and is also connected through the resistor 107 to the terminal T1. The node A between the two lithium-ion cells 101 and 102 is connected through the resistor 108 to the terminal T2. The lower-potential end of the series of the two lithium-ion cells 101 and 102 is connected through the FET 103 to the negative terminal 106, and is also connected to the terminal T3. Note that the terminal T3 is grounded within the lithium-ion cell protection circuit 200, and therefore the voltage at the lower-potential end of the series of the two lithium-ion cells 101 and 102 equals the ground level. The charge control FET control circuit 104 has its input connected to the terminal T4, and has its output connected to the gate of the charge control FET 103. The chip capacitor 109 has its one end connected to the terminal T5, and has its other end grounded.
The lithium-ion cell protection circuit 200 consists of an overcharge detection circuit 1 and a delay setting circuit 7. The overcharge detection circuit 1 is composed of two comparators 11 and 12, two constant voltage sources 13 and 14 whose output voltages are V.sub.ref1 and V.sub.ref2 respectively, and an OR circuit 15. The comparator 11 has its non-inverting input terminal (+) connected to the terminal T1, and has its inverting input terminal (-) connected to the higher-potential end of the constant voltage source 13, whose lower-potential end is connected to the terminal T2. The comparator 12 has its non-inverting input terminal (+) connected to the terminal T2, and has its inverting input terminal (-) connected to the higher-potential end of the constant voltage source 14, whose lower-potential end is grounded. The OR circuit 15 receives as its inputs the outputs of the comparators 11 and 12, and the output of the OR circuit 15 is used as the output of the overcharge detection circuit 1.
As a result of the above-described interconnection, the overcharge detection circuit 1 operates as follows. When the voltage across either of the lithium-ion cells 101 and 102 exceeds a predetermined voltage (the predetermined voltage is V.sub.ref1 for the lithium-ion cell 101 and V.sub.ref2 for the lithium-ion cell 102), that is, when overcharge starts, the overcharge detection circuit 1 outputs a high level (this high-level output is hereafter referred to as the "overcharge detection signal").
On the other hand, the delay setting circuit 7 is composed of a comparator 71, constant current sources 72 and 76, an npn-type transistor 73, an inverter 74, and a constant voltage source 75 whose output voltage is V.sub.0. The constant current source 72 is connected to the OR circuit 15, and is so configured as to be turned on when it receives the overcharge detection signal. The output of the constant current source 72 is connected to the terminal T5 and also to the collector of the transistor 73. The inverter 74 has its input connected to the output of the overcharge detection circuit 1 (i.e. the OR circuit 15), and has its output connected to the base of the transistor 73. The emitter of the transistor 73 is grounded. The comparator 71 has its non-inverting input terminal (+) connected to the node B between the constant current source 72, the terminal T5, and the collector of the transistor 73, and has its inverting input terminal (-) connected to the higher-potential end of the constant voltage source 75, whose lower-potential end is grounded. The constant current source 76 is so configured as to be turned on when it receives a high-level output from the comparator 71, and its output is connected to the terminal T4.
As a result of the above-described interconnection, the delay setting circuit 7 operates as follows. When the delay setting circuit 7 receives the overcharge detection signal from the overcharge detection circuit 1, the npn-type transistor 73 is turned off. This causes the constant current source 72 to start supplying a current to the terminal T5 and thereby start charging the chip capacitor 109. As time passes, the voltage across the chip capacitor 109 rises, and, when this voltage exceeds the voltage V.sub.0, the comparator 71 outputs a high level, causing the constant current source 76 to supply a predetermined current to the terminal T4 (this predetermined current is hereafter referred to as the "charge inhibition signal"). That is, in response to the overcharge detection signal from the overcharge detection circuit 1, the delay setting circuit 7 outputs the charge inhibition signal with a predetermined delay t (t=CV.sub.0 /I.sub.0) that depends on the current I.sub.0 supplied from the constant current source 72, the capacitance C of the chip capacitor 109, and the output voltage V.sub.0 of the constant voltage source 75.
Further, the input of the charge control FET control circuit 104 is connected to the terminal T4, so that, when the charge control FET control circuit 104 receives the charge inhibition signal from the delay setting circuit 7, it outputs a low level, and thereby causes the charge control FET 103 to be turned off. As a result, the lithium-ion cells 101 and 102 are cut off from a charging unit that is connected between the positive and negative terminals 105 and 106, and thereby the charging of the lithium-ion cells 101 and 102 is stopped. In this way, the lithium-ion cells 101 and 102 are protected against overcharge.
The purpose of securing a delay as described above between the detection of overcharge and the inhibition of the charging of the lithium-ion cells is to cancel erroneous activation of the overcharge detection circuit 1 by external noises.
As shown in FIG. 5, the above-described lithium-ion power supply unit is formed as a hybrid IC (integrated circuit) that has its constituent elements shown in FIG. 4, such as the chip capacitor 109 and the lithium-ion cell protection circuit 200, mounted on a single substrate 300. As a result, this lithium-ion power supply unit tends to suffer from a short circuit in the chip capacitor 109 that is caused by a crack in the chip capacitor 109, by a defect in the wiring pattern on the substrate 300, or for some other reason.
A disadvantage of the above-described conventional lithium-ion cell protection circuit 200 is that, when a short circuit occurs in the chip capacitor 109, the voltage of the non-inverting input terminal (+) of the comparator 71 of the delay setting circuit 7 is fixed at the ground level, and thus the comparator 71 never outputs a high level even if the overcharge detection circuit 1 outputs the overcharge detection signal. This means that the delay setting circuit 7 never outputs the charge inhibition signal, and accordingly, even in case of overcharge, the charging of the lithium-ion cells 101 and 102 is continued until they are brought into such hazardous condition that they start emitting smoke or even burning.